A courting peacock's eye spots are a captivating sight, seemingly floating in place on an upright, shimmering iridescent train. And now, researchers in North America have uncovered the avian show-off's tricks.
Roslyn Dakin from the University of British Columbia, Canada, and colleagues used high-speed video, microscopy and mathematical modelling to unpick the physics behind the peacock's "train rattle", and found the display to be a full audiovisual experience.
Charles Darwin, writing in a book on sexual selection in the 19th century, wasn't convinced there was anything beneficial about the Indian peafowl's (Pavo cristatus) train rattle: "Peacocks […] rattle their quills together, and the vibratory movement apparently serves merely to make noise, for it can hardly add to the beauty of their plumage."
Last year, researchers discovered peacock tails produce low, rumbling infrasound which is heard by other members of their species. Yet the biomechanics of peacock feathers is poorly understood.
The success of a courting peacock doesn't seem to rely on the length of his feathers or the number of eye spots his tail is adorned with. Rather, the females – peahens – tend to choose males with the most iridescent display, where different angles produce different colours.
Feathers are usually made of a central shaft called a rachis which has a series of branches, called barbs, fused along it. Barbs are further split into "barbules" which have tiny hooks that latch onto the neighbouring barb.
Downy feathers lack these hooks, so the barbs are free and fluffy. Flying feathers, though, are loaded with these hooks to latch the barbs together. If you pull the barbs of a feather apart, you can usually smooth them together again.
Dakin and her team looked peacock eye spot feathers under a microscope for clues to their hypnotic display. Indeed, they saw the green barbs up the feather's rachis were downy and hook-free, while the barbs that comprised the eye spot were heavily hooked, like a flying feather, and kept those barbs latched together strongly.
This meant that when the feather was vibrated, the hookless barbs could flick around, and shimmer, freely. But the relative denseness of the eye spot meant it stayed still.
When Dakin and her colleagues filmed 14 adult male peacocks in full display mode, they noticed they rattled their trains at frequencies between 22 and 28 times per second, with an average of 25.6.
This, they calculated, was the feather's resonance. In other words, that particular frequency sent a wave up the rachis which maximised the amplitude at some parts (anti-nodes) while keeping intervening points still (called nodes). The eye spots, they noted, tended to sit near nodes – confirmed when they took eye spot feathers into the lab and shook them at a frequency of 25.6 waves per second.
And as it takes a lot of energy to hoist a train, rattle it and strut around – sometimes for 25 minutes at a time – the team decided to see if peacocks were getting the most for their energy buck.
A little bit of nifty modelling showed the most efficient frequencies were between 22 and 27 cycles per second, pretty much the exact range they observed peacocks rattling at. The frequencies also generated infrasound – in other words, two communication signals for the price of one.
More work is needed to see if peahens use audiovisual features to pick a mate, and benefits they might gain, the researchers write: "or whether Darwin was right and it is all just noise."
The secret signal in a peacock's tail
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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